It is well recognized that the efficiency of a transistor switching circuit is related to the rise and fall times of the transitions between switching states. A fast rise and fall time reduces that period of time when the circuit is switching and the power dissipation is the greatest. Because an important commercial consideration of transistors is its maximum operating frequency, there is an ongoing effort to develop faster solid state structures and circuits.
Because the fall or turn-off time, of a bipolar transistor is in many instances longer than its rise time, special attention and emphasis has been placed on decreasing the turn-off time of transistors. Power transistors with large output geometries generally represent major power dissipation elements in circuits. This is a result of the capacitance between the base and collector junction which stores enough charge to keep the transistor conducting for a short period of time even though the base drive has been removed. In fabricating solid state structures, it is a conventional practice to diffuse a small amount of gold into the device structure to enhance the recombination of holes and electrons and thereby reduce the time in which the base charge is dissipated.
It is also well known in the art that the turn-off time of transistors can be decreased by the use of a resistor connected to the base for allowing the excess base charge to leak through the resistor to ground. While this approach decreases the turn-off time, it also lowers the input impedance of the circuit and thus requires the input circuit to supply more current.
Active circuits have been developed which further improve turn-off times by reversing the base drive current of the transistor and drawing the capacitive charge out of its base-collector junction. One conventional technique for reversing the base drive involves the connection of an auxiliary transistor to the base of the power device, and capacitively coupling the auxiliary transistor with respect to the circuit input drive. Thus, when the power device drive signal is removed, the auxiliary transistor turns on momentarily to draw the base charge out of the power device. While this type of technique requires no sustaining DC current, the capacitor component requires a large amount of substrate space. Moreover, the timing capacitor and resistor values are critical in producing a time constant which maintains the auxiliary transistor active for a predetermined period of time.
Another approach for drawing the base charge out of a power device involves the use of an auxiliary transistor also connected to the power device base to reverse the base current, but which auxiliary transistor is DC coupled with regard to the input signal and is always conductive when the power device is in its off state. While this type of circuit is effective in drawing out all the base charge, and requires little substrate space, it reduces the overall circuit response as it must be deactivated before the power device can be turned on.
It is seen, therefore, that there is a need for a circuit which further improves the turn-off time of a bipolar semiconductor device, and yet overcomes the shortcomings of the techniques heretofore employed.
There is a further need for a transistor turn-off circuit which has the advantage of an AC coupled turn-off circuit, insofar as it is active momentarily, but which is DC coupled and requires very little DC current.
There is also a concomitant need for a transistor turn-off circuit with improved efficiency when operating at high switching speeds.